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United States Patent |
5,300,814
|
Matsumoto
,   et al.
|
April 5, 1994
|
Semiconductor device having a semiconductor substrate with reduced step
between memory cells
Abstract
A semiconductor device comprising a semiconductor substrate, a plurality of
memory cell regions each having a plurality of memory cells disposed on
the semiconductor substrate, a word line formed in a first level above the
semiconductor substrate, a bit line formed in a second level above the
first level, and a backing line having a lower resistance than the word
line and formed in a third level above the second level. A dummy bit line
is formed in the second level outside the memory cell region so as to
reduce the step formed at the periphery of the memory cell region. The
dummy bit line is also used to interconnect the word line and the backing
line so that an electrical connection therebetween is stabilized.
Inventors:
|
Matsumoto; Susumu (Hirakata, JP);
Hashimoto; Shin (Hirakata, JP);
Yamada; Toshio (Kadoma, JP);
Nakata; Yoshiro (Ikoma, JP)
|
Assignee:
|
Matsushita Electric Industrial Co., Ltd. (Osaka, JP)
|
Appl. No.:
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915898 |
Filed:
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July 17, 1992 |
Foreign Application Priority Data
Current U.S. Class: |
257/758; 257/296; 257/659; 257/920; 257/E23.142; 257/E23.145; 257/E23.151; 257/E27.081 |
Intern'l Class: |
H01L 023/48; H01L 029/46 |
Field of Search: |
257/758,756,296,306,920,659
|
References Cited
U.S. Patent Documents
4679171 | Jul., 1987 | Logwood et al. | 257/659.
|
5110762 | May., 1992 | Nakahara et al. | 257/758.
|
Foreign Patent Documents |
97665 | May., 1985 | JP | 257/296.
|
64-81358 | Mar., 1989 | JP.
| |
Primary Examiner: Hille; Rolf
Assistant Examiner: Limanek; Robert
Attorney, Agent or Firm: Ratner & Prestia
Claims
What is claimed is:
1. A semiconductor device, comprising:
a semiconductor substrate,
a plurality of memory cell regions each having a plurality of memory cells
disposed on said semiconductor substrate,
a first conductive layer formed in a first level located above said
semiconductor substrate;
a second conductive layer formed in a second level located above said first
level; and
a third conductive layer formed in a third level located above said second
level, said third conductive layer having a lower resistance than said
first conductive layer:
wherein said second conductive layer comprises a wiring portion and a dummy
portion, said dummy portion being disposed outside said memory cell
regions and interconnecting said first conductive layer and said third
conductive layer.
2. A semiconductor device comprising a semiconductor substrate, a first
conductive layer formed in a first level located above said semiconductor
substrate, a second conductive layer formed in a second level located
above said first level, and a third conductive later formed in a third
level located above said second level
wherein said second conductive layer comprises a first portion and a second
portion electrically insulated from each other
said second portion of said second conductive layer interconnects said
first conductive layer;
said first conductive layer includes a word line connected to said memory
cells;
said first portion of said second conductive layer includes bit line
connected to said memory cells;
said second portion of said second conductive layer is located outside said
memory cell regions and includes a dummy bit line; and
said third conductive layer includes a backing line having a lower
resistance than said first conductive layer.
3. A semiconductor device comprising a semiconductor substrate, a first
conductive layer formed in a first level located above said semiconductor
substrate, a second conductive layer formed in a second level located
above said first level, and a third conductive layer formed in a third
level located above said second level, wherein
said second conductive layer comprises a first portion and a second portion
electrically insulated from each other;
said second portion of said second conductive layer interconnects said
first conductive layer;
said first conductive layer includes a word line connected to said memory
cells;
said first portion of said second conductive layer includes a bit line
connected to said memory cells;
said second portion of said second conductive layer is located outside said
memory cells and includes a dummy bit line; and
said third conductive layer includes a backing line having a lower
resistance than said first conductive layer,
said semiconductor device further comprising a fourth conductive layer
between said second level and said third level, said fourth conductive
layer including a plate electrode portion located in said memory cell
regions, and being electrically isolated from said first conductive layer
and said second conductive layer.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention:
The present invention relates to a semiconductor device, and more
particularly relates to a semiconductor device in which a signal delay at
a lower level line made of a material having a comparatively high
resistance and high melting point, such as polysilicon, is compensated by
an upper level line made of a material having a comparatively low
resistance, such as aluminum. The present invention also relates to a
process for fabricating such a semiconductor device.
2. Description of the Prior Art:
Since the packing density of a semiconductor device in which a number of
semiconductor elements are integratedly formed on one chip has been
enlarged, the length of wiring interconnecting the elements on the chip
has been increased. On the other hand, the width of the wiring has been
reduced so as to enhance the packing density. Generally, a long and narrow
wiring has a high resistance. Wiring having a high resistance causes a
problem of signal delay, which reduces the operation speed of the
semiconductor device.
In a dynamic random access memory (DRAM), material having a low resistance
and low melting point such as aluminum can not be used for a word line
because it is not durable in the fabricating process of the semiconductor
device. Instead, a material having a relatively high resistance and high
melting point such as polysilicon and polycide is used. In such a DRAM, in
order to prevent the problem of signal delay that occurs at the word line,
another wiring made of aluminum is formed above the word line so as to run
substantially in parallel with the word line. This aluminum wiring, called
a "backing line", is connected to the word line at a plurality of points.
A conventional semiconductor device provided with the backing line will be
described in detail. FIGS. 5A and 5B show partial sectional views of such
a conventional semiconductor device. The semiconductor device has a
structure of stacked capacitor cells. Referring to FIG. 5A, a part of the
semiconductor device shown as B (right side of the dash-dot line in the
figure) is a memory cell region having a plurality of memory cells. The
other part of the semiconductor device shown as A (left side of the
dash-dot line in the figure) is a memory cell peripheral region
surrounding the memory cell region B. In the memory cell region B, a
plurality of MOSFETs and memory capacitors connected to the respective
MOSFETs are arranged in a matrix.
FIG. 6 schematically shows four memory cell regions B and the memory cell
peripheral regions A surrounding these memory cell regions B. Each memory
cell region B includes 256.times.64 memory cells arranged in a matrix.
Each of the memory cells is connected to a word line 4 and a bit line 8.
Thus, totally 256 word lines 4 and 64 bit lines 8 run through one memory
cell region B. Each word line 4 and an aluminum backing line 15 running in
parallel with the word line 4 are connected to each other in the memory
cell peripheral region A, not in the memory cell region B. Sense
amplifiers are also disposed in the memory cell peripheral region A.
Referring to FIGS. 5A and 5B, the structure of the conventional
semiconductor device will be described. The semiconductor device comprises
a semiconductor substrate 1, a level of the word lines 4, a level of the
backing lines 15, and a level of bit lines 8 disposed between the above
two levels. The bit lines 8 run transversely to the direction of the word
lines 4 and the backing lines 15 (FIG. 6). Each bit line 8 is composed of
a polysilicon film 8a as a lower layer and a silicide film 8b as an upper
layer.
A field oxide film 2 is formed between the semiconductor substrate 1 and
the word lines 4. As shown in FIG. 5B, each word line 4 is covered with an
upper insulating film 5 on the top surface thereof and with side-wall
insulating films 6 on the sides thereof. A first interlevel insulating
film 18 is formed between the word lines 4 and the bit lines 8 for
insulating them from each other. The bit lines 8 are electrically
connected to sources of transistors in the memory cell region B through
respective contact holes (not shown) formed through the first interlevel
insulating film 18.
A second interlevel insulating film 9 is formed over the first interlevel
insulating film 18 so as to cover the bit lines 8. On the second
interlevel insulating film 9 in the memory cell region B, memory
capacitors comprising storage nodes 10, capacitor insulating films 11, and
a plate electrode 12a are formed. Each storage node 10 is connected to a
drain (not shown) of the corresponding transistor.
A third interlevel insulating film 13 is formed over the second interlevel
insulating film 9 so as to cover the plate electrode 12a. In the memory
cell peripheral region A, contact holes 17 having a high aspect ration are
formed through the interlevel insulating films 13, 9, and 18 and the upper
insulating film 5 to reach the word lines 4. Each contact hole 17 has a
tungsten (W) plug 14 formed therein. Thus, each aluminum backing line 15
having a low resistance is electrically connected to the corresponding
word line 4 through the tungsten plug 14. The backing line 15 runs above
the word line 4 substantially in parallel therewith.
As described above, the memory cell region B includes the bit lines 8
(thickness: about 240 nm), the storage nodes 10 (thickness: about 500 nm),
the capacitor insulating films 11 (thickness: about 5 nm), and the plate
electrode 12a (thickness: about 100 nm), which are not formed in the
memory cell peripheral region A. Therefore, the third interlevel
insulating film 13 in the memory cell region B is formed in a higher
position than that in the memory cell peripheral region A by the total
thickness of the above layers (about 845 nm), producing a step of that
height at the boundary between the memory cell region B and the memory
cell peripheral region A. This step causes the following problems in the
photolithography and etching processes:
(1) Since the height of the step exceeds the allowable depth of focus for
the photolithography (normally, no more than about 800 nm), a precise
focusing on both the memory cell region B and the memory cell peripheral
region A is not possible. Thereby, it is difficult to form a fine resist
pattern on the semiconductor device.
(2) An aluminum film formed by sputtering on the vertical side of the step
portion is thinner than that on the flat portion. Thereby, the reliability
of the aluminum backing line 15 including such a thin film portion is
lowered.
(3) The thickness of the aluminum film in the direction vertical to the
substrate 1 is larger at the step portion than at the flat portion.
Therefore, after the aluminum film is etched by a high anisotropic etching
method, some aluminum tends to be left behind on the side of the step
portion. Such aluminum left behind on the side of the step portion may
cause a short circuit between the aluminum backing lines 15.
(4) Tungsten (or polysilicon) is deposited on the entire surface of the
third interlevel insulating film 13 and then etched back so as to form a
CVD tungsten plug (or a CVD polysilicon plug) inside the contact hall 17
by a blanket method. After the etching back, some of the tungsten (or the
polysilicon) tends to be left behind on the step portion.
Further, in the conventional semiconductor devices, the aspect ratio of the
contact hole 17 for connecting the aluminum backing line 15 with the word
line 4 is high (about 3.5 to 5.0). It is difficult to form the tungsten
plug 14 through the contact hole 17 having a high aspect ratio.
SUMMARY OF THE INVENTION
The semiconductor device of this invention including a semiconductor
substrate and a plurality of memory cell regions each having a plurality
of memory cells disposed on the semiconductor substrate, comprises a first
conductive layer formed in a first level located above the semiconductor
substrate; a second conductive layer formed in a second level located
above the first level; and a third conductive layer formed in a third
level located above the second level, the third conductive layer having a
lower resistance than the first conductive layer: wherein the second
conductive layer comprises a wiring portion and a dummy portion, the dummy
portion being disposed outside the memory cell regions and interconnecting
the first conductive layer and the third conductive layer.
In another aspect of the present invention, a process for fabricating a
semiconductor device is provided. The process comprises the steps of:
forming a word line on a semiconductor substrate; forming a first
insulating film on the semiconductor substrate so as to cover the word
line; forming a first contact hole through the first insulating film to
reach the word line; forming a conductive film on the first insulating
film; patterning the conductive film to form a bit line and a dummy bit
line electrically separated from each other, the dummy bit line being
connected to the word line through the first contact hole; forming a
second insulating film on the first insulating film so as to cover the bit
line and the dummy bit line; forming a second contact hole through the
second insulating film to reach the dummy bit line; and forming a backing
line on the second insulating film, the backing line being connected to
the dummy bit line through the second contact hole.
According to the semiconductor device of the present invention, the second
conductive layer formed in the second level includes the first portion
functioning as a wiring and the second portion that does not function as a
wiring but formed as a dummy outside the memory cell regions. As a result,
a step produced at the periphery of each memory cell region by the
existence of the first portion of the second conductive layer is reduced.
Hence, decrease in the production yield caused by the step is prevented.
Further, the first conductive layer and the third conductive layer are
connected to each other, not through one contact hole as in the
conventional semiconductor device, but indirectly through the second
portion of the second conductive layer located outside the memory cell
regions. As a result, a stable electrical connection between the first
conductive layer and the third conductive layer can be achieved.
Especially, when the semiconductor device of the present invention
comprises a plurality of memory cell regions each having a plurality of
memory cells disposed on the semiconductor substrate, the word line as the
first conductive layer, the bit line as the first portion of the second
conductive layer, the dummy bit line as the second portion of the second
conductive layer, and the backing line as the third conductive layer, a
semiconductor device having a high integration level in which signal delay
at the word line is compensated by the backing line can be obtained with a
high production yield.
Thus, the invention described herein makes possible the advantage of
providing a semiconductor device in which a step at the boundary between
the memory cell region and the memory cell peripheral region can be
reduced and the electrical connection between the word line and the
backing line can be stabilized. A process for fabricating such a
semiconductor device is also provided.
These and other advantages of the present invention will become apparent to
those skilled in the art upon reading and understanding the following
detailed description with reference to the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partial structural plan view of a semiconductor device
according to the present invention;
FIGS. 2A and 2B are partial sectional views of the semiconductor device of
FIG. 1, taken along line X--X of FIG. 1 for FIG. 2A and line Y--Y for FIG.
2B, at an intermediate stage of a fabricating process according to the
present invention;
FIGS. 3A and 3B are partial sectional views of the semiconductor device of
FIG. 1, taken along line X--X of FIG. 1 for FIG. 3A and line Y--Y for FIG.
3B, at another intermediate stage of the fabricating process according to
the present invention;
FIGS. 4A and 4B are partial sectional views of the semiconductor device of
FIG. 1, taken along line X--X of FIG. 1 for FIG. 4A and line Y--Y for FIG.
4B;
FIGS. 5A and 5B are partial sectional views of a conventional semiconductor
device;
FIG. 6 is a schematic plan view of part of a semiconductor device showing
four memory cell regions and their peripheral regions; and
FIG. 7 is a table for comparing the present invention with a prior art.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIGS. 4A and 4B are partial sectional views of a semiconductor device,
which is a DRAM, according to the present invention. In FIG. 4A, a part of
the semiconductor device shown as B (right side of the dash-dot line in
the figure) is a memory cell region having a plurality of memory cells.
The other part of the semiconductor device shown as A (left side of the
dash-dot line in the figure) is an memory cell peripheral region. In the
memory cell region B, a plurality of MOSFETs and a plurality of memory
capacitors connected to the respective MOSFETs are arranged in a matrix.
This planar arrangement is the same as that of the conventional
semiconductor device, which is schematically shown in FIG. 6.
FIG. 1 is a plane layout of the memory cell peripheral region A of the
semiconductor device of the present invention. A section taken along line
X--X in this figure is shown as FIG. 4A and a section taken along line
Y--Y as FIG. 4B. Referring to FIG. 1, the areas patterned with oblique
lines show word lines 4. In the memory cell region B, bit lines 8 run
transversely to the direction of the word lines 4. Each bit line 8 is
composed of a polysilicon film 8a as a lower layer and a silicide film 8b
as an upper layer. Contact holes 7a are formed in the memory cell region B
for connecting the bit lines 8 to corresponding source/drain regions of
transistors (not shown).
In the memory cell peripheral region A, dummy bit lines 19 corresponding to
the number of the word lines 4 are formed. Each dummy bit line 19 is
composed of a polysilicon film 19a as a lower layer and a silicide film
19b as an upper layer. In the region A, some portions of the word lines 4
are wider than other portions thereof, and on each of such widened
portions of the word lines 4, a large contact hole 7b is formed for
connecting the word line 4 with the corresponding dummy bit line 19. As
shown in FIG. 1, the location of the widened portion of one word line 4 is
deviated from that of the adjacent word line 4 in the direction of the
length of the word line 4, so that the pitch of the word lines 4 does not
increase in the memory cell region B by the formation of the widened
portions.
Aluminium backing lines 15 are formed substantially right above the
corresponding word lines 4, though they are omitted in FIG. 1 for
simplification. Contact holes 16 for connecting the aluminum backing lines
15 and the dummy bit lines 19 are formed near the positions of the contact
holes 7b.
Referring to FIGS. 4A and 4B, the sectional structure of the semiconductor
device of the present invention will be described. The semiconductor
device comprises a semiconductor substrate (a p-type silicon substrate) 1,
a first conductive layer (the word lines 4) constituting a first level
formed above the semiconductor substrate 1, a second conductive layer (the
bit lines 8 and the dummy bit lines 19) constituting a second level formed
above the first level, and a third conductive layer (the aluminum backing
lines 15) constituting a third level formed above the second level.
The sectional structure of each of the first to third conductive layers
will be described. The first conductive layer comprises the word lines 4
each of which interconnects gates of the memory cell transistors in the
memory cell region B and a column decoder (not shown). Referring to FIG.
4A, a portion of the word line 4 located above a channel C of the memory
cell transistor functions as the gate electrode of the memory cell
transistor. A thin gate insulating film 3 is formed between the word line
4 and the channel C.
The second conductive layer (thickness: 240 nm) comprises the bit lines 8
in the memory cell region B, which are connected to the sources of the
memory cell transistors, and the dummy bit lines 19 in the memory cell
peripheral region A.
The third conductive layer comprises the low-resistance backing lines 15
for compensating signal delay at the word lines 4. The backing lines 15
run above the word lines 4 substantially in parallel with the word lines
4. The third conductive layer is electrically connected to the word lines
4 at a plurality of places in the memory cell peripheral region A, not in
the memory cell region B.
The structure of the semiconductor device of the present invention will be
described in detail. A field oxide film 2 is formed between the
semiconductor substrate 1 and the word lines 4. The field oxide film 2
includes a LOCOS film, for example. The LOCOS film is formed by
selectively oxidizing prescribed portions of the surface of the
semiconductor substrate 1. At the other portions of the surface thereof
remained uncovered with the field oxide film 2 (active regions),
semiconductor elements such as transistors are formed. In the
semiconductor device of the present invention, a MOSFET having a channel C
in the substrate 1 is formed as the semiconductor element.
As shown in FIG. 4B, each word line 4 is covered with an upper insulating
film 5 on the top surface thereof and with side-wall insulating films 6 on
the sides thereof. A first interlevel insulating film 18 is formed between
the word lines 4 and the bit lines 8 for insulating them from each other.
The unevenness produced by the pattern of the word lines 4 is made smooth
by the planarized first interlevel insulating film 18.
On the smooth surface of the first interlevel insulating film 18, the bit
lines 8 and the dummy bit lines 19 are formed. The bit lines 8 are
electrically connected to the sources of the transistors through the
contact holes 7a (shown in FIG. 1) in the memory cell region B. The dummy
bit lines 19, which are formed in the memory cell peripheral region A, are
electrically connected to the word lines 4 through the contact holes 7b
formed through the first interlevel insulating film 18. Preferably, the
dummy bit lines 19 are formed near the boundary between the memory cell
region B and the memory cell peripheral region A, as shown in FIG. 4A. In
this embodiment, the right end of the dummy bit line 19 formed nearest to
the memory cell region B is located about 100 nm to the right of the
dash-dot line in FIG. 4A. However, it may be located about 1000 nm to the
left of the dash-dot line so as to achieve the same effect of reducing the
step at the boundary.
A second interlevel insulating film 9 is formed over the first interlevel
insulating film 18 so as to cover the bit lines 8 and the dummy bit lines
19. On the second interlevel insulating film 9 in the memory cell region
B, memory capacitors including storage nodes 10, capacitor insulating
films 11, and a plate electrode 12a are formed. Each storage node 10 is
connected to the corresponding transistor. The on/off state of the
transistor is controlled according to the potential of the corresponding
word line 4. When the transistor is on, the bit line 8 and the storage
node 10 are electrically connected through the transistor.
On the second interlevel insulating film 9 in the memory cell peripheral
region A, a dummy plate electrode 12b is formed. The plate electrode 12a
and the dummy plate electrode 12b constitute a fourth conductive layer. A
third interlevel insulating film 13 is formed over the second interlevel
insulating film 9 so as to cover the plate electrode 12a and the dummy
plate electrode 12b. In the memory cell peripheral region A, the contact
holes 16 are formed through the second and third interlevel insulating
films 9 and 13 at the portions where the dummy plate electrodes 12b are
not formed therebetween. Each contact hole 16 has a tungsten (W) plug 14
formed therein. The aluminum backing lines 15 are electrically connected
with the corresponding dummy bit lines 19 through the tungsten plugs 14.
In the semiconductor device of the present invention as described above,
the dummy bit lines 19 are formed in the memory cell peripheral region A.
Therefore, the step on the surface of the third interlevel insulating film
13 produced at the boundary between the memory cell region B and the
memory cell peripheral region A is reduced by about 240 nm, compared with
that in the conventional semiconductor device. This is further reduced by
the formation of the dummy plate electrode 12b having a thickness of 100
nm in the memory cell peripheral region A. It is not necessary to form the
dummy plate electrode 12b when the step at the boundary can be
sufficiently reduced without the formation of the dummy plate electrode
12b.
FIG. 7 is a table of the thickness of each layer of both the memory cell
region B and the memory cell peripheral region A, together with the height
of the step produced between the regions B and A, in the semiconductor
device of the present invention as shown in FIGS. 4A and 4B, compared with
those in the conventional semiconductor device as shown in FIGS. 5A and
5B. As is apparent from the table, the step can be significantly reduced,
from 845 nm in the conventional semiconductor device to 505 nm in the
semiconductor device of the present invention.
The process for fabricating the semiconductor device according to the
present invention will be described.
In this embodiment, patterning is performed according to a 0.4 .mu.m design
rule, so as to form fine patterns preferably by excimer-laser lithography.
An electron beam direct writing technique or an i-line phase shift
technique may also be employed instead of the excimer-laser lithography.
First, referring to FIGS. 2A and 2B, the surface of the semiconductor
substrate 1 is selectively oxidized by the LOCOS method so as to form the
field oxide film 2 on a prescribed portion of the surface. The other
portion thereof on which the field oxide film 2 is not formed functions as
an active region.
The surface of the active region is oxidized so as to form the gate
insulating film 3. Then, polysilicon is deposited over the field oxide
film 2 and the gate insulating film 3 by CVD to form a polysilicon film.
Subsequently, an oxide film is formed on the polysilicon film by CVD. The
oxide film and the polysilicon film are then patterned, thus to form the
word lines 4 and the upper insulating films 5. On the sides of each of the
word lines 4 and the upper insulating films 5, the side-wall insulating
films 6 are formed by a known method. The word lines 4 may also be formed
by patterning a refractory metal film or a refractory metal silicide film
having a high melting point, or a polycide film composed of a polysilicon
film as a lower layer and a silicide film as an upper layer.
Then, impurity ions are implanted in the active region of the semiconductor
substrate 1 by ion implantation so that the source/drain of the transistor
(not shown) can be formed by self-alignment with the word line 4.
The first interlevel insulating film 18 is formed over the semiconductor
substrate 1 by CVD so as to cover the word lines 4. Then, the surface of
the first interlevel insulating film 18 is made smooth employing an etch
back technique or a reflow technique. When the reflow technique is
employed, the first interlevel insulating film 18 is composed of a
non-doped oxide film as a lower layer and a BPSG (boro-phospho silicate
glass) film as an upper layer. Since, the BPSG film tends to reflow at a
low temperature compared with the non-doped oxide film, the upper surface
of the first interlevel insulating film 18 can be made smooth by heat
treatment to cause the BPSG film to reflow.
Then, the contact holes 7a and 7b are formed in prescribed portions of the
first interlevel insulating film 18. The contact hole 7a (shown only in
FIG. 1) is formed in the memory cell region B so as to connect the bit
line 8 to the active region (source/drain) of the transistor. On the other
hand, the contact hole 7b is formed in the memory cell peripheral region A
so as to connect the dummy bit line 19 to the word line 4. The contact
hole 7b is formed on the widened portion of the word line 4, so that the
precise mask alignment at the photolithography can be easily performed.
The contact holes 7a and 7b are simultaneously formed. The contact hole 7b
has a depth of about 600 to 800 nm, a diameter of about 350 to 400 nm, and
an aspect ratio of 1.5 to 2.3.
Then, a polysilicon film and a silicide film are sequentially formed and
patterned together so as to form the bit lines 8 and the dummy bit lines
19 having a polycide structure simultaneously. Each of the dummy bit lines
19 is separated into a plurality of sections, and each section has a
function of connecting the corresponding word line 4 and the aluminum
backing line 15. The distance between the adjacent dummy bit lines 19 is
0.4 .mu.m, which is the minimum size allowable for the design rule
applied. At this stage, each of the dummy bit lines 19 is connected to the
respective word line 4 through the contact hole 7b. This connection can be
easily performed because the aspect ratio of the contact holes 7b is very
small, compared with that of the contact holes 17 of the conventional
semiconductor device shown in FIGS. 5A and 5B. A single-layer wiring such
as a polysilicon line, a refractory metal line having a high melting
point, or a refractory metal silicide line having a high melting point can
also be used as the bit line, instead of the polycide line.
Referring to FIGS. 3A and 3B, the second interlevel insulating film 9 is
formed over the bit lines 8 and the dummy bit lines 19. Then, the surface
of the second interlevel insulating film 9 is made smooth as in the case
of the first interlevel insulating film 18. A contact hole (not shown) is
formed through the second and first interlevel insulating films 9 and 18
in the memory cell region B. Then, a phosphorus-doped polysilicon film (P
concentration: a few percent) is formed on the second interlevel
insulating film 9 and patterned so as to form the storage nodes 10 which
are connected to the corresponding active regions of the transistors. The
capacitance insulating films 11 made of SiO.sub.2 /Si.sub.3 N.sub.4
/SiO.sub.2 (ONO) are formed over the surfaces of the storage nodes 10. A
refractory metal such as tungsten or refractory metal silicide may also be
used as a material for the storage nodes 10.
The plate electrode 12a is formed on the second interlevel insulating film
9 so that the storage nodes 10 and the capacitance insulating film 11 are
buried therein. At this time, the dummy plate electrode 12b is also formed
over the surface of the second interlevel insulating film 9 in the memory
cell peripheral region A, except for the portions thereof where the
contact holes 16 are formed, as well as the marginal portions for
photolithography alignment at the formation of the contact holes 16. The
plate electrode 12a and the dummy plate electrode 12b are formed by
patterning the same conductive film, for example, the phosphorus-doped
polysilicon film. A refractory metal such as tungsten or refractory metal
silicide may also be used as a material for the plate electrode 12a and
the dummy plate electrode 12b. In this embodiment, the thickness of these
electrodes 12a and 12b is 100 nm. Also, in this embodiment, as shown in
FIG. 3A, the plate electrode 12a and the dummy plate electrode 12b are
connected at the step portion (the boundary between the memory cell region
B and the memory cell peripheral region A). However, the reduction of the
step can also be achieved when these electrodes 12a and 12b are not
connected.
The third interlevel insulating film 13 is formed over the second
interlevel insulating film 9 so that the plate electrode 12a and the dummy
plate electrode 12b are buried therein. The third interlevel insulating
film 13 is made smooth by heat treatment, etching back, and the like. A
SOG (spin-on-glass) film may be used as the third interlevel insulating
film 13. In the memory cell peripheral region A, the contact holes 16 are
formed by lithography and etching through the second and third interlevel
insulating films 9 and 13 at the portions where the dummy plate electrodes
12b are not formed. Each of the contact holes 16 has a depth of about 800
to 1000 nm, a diameter of about 350 to 400 nm, and an aspect ratio of 2 to
2.9. As indicated above, the aspect ratio of the contact hole 16 is very
small, compared with that of the contact hole 17 for connecting the word
line 4 and the aluminum backing line 15 of the conventional semiconductor
device. Further, the contact hole 16 is formed on the dummy bit line 19
which is wider than the widened portion of the word line 4. Therefore, the
precise mask alignment at photolithography can be easily performed.
The tungsten (W) plug 14 is formed inside the contact hole 16 by a
selective CVD method. Then, the aluminum backing lines 15 are formed on
the third interlevel insulating film 13 by sputtering and aluminum
etching. In this way, each of the aluminum backing lines 15 is
electrically connected to the word line 4 through the tungsten plug 14 and
the dummy bit line 19. The tungsten plug 14 can be easily formed inside
the contact hole 16 since the aspect ratio of the contact hole 16 is
small. Before the formation of the tungsten plug 14, a barrier metal made
of TiN/Ti, for example, may be formed on the bottom surface of the contact
hole 16. Then, the tungsten plug 14 can be formed by a blanket W-CVD
method. Alternatively, the aluminum backing line 15 may be connected with
the dummy bit line 19 directly without the formation of the plug 14.
As described above, according to the fabricating process of the present
invention, the aluminum backing line 15 can be connected to the word line
4 by an effective method of forming two kinds of contact holes both having
a low aspect ratio. Further, since the conductive material is formed
inside the contact hole 16, a contact failure between the aluminum backing
line 15 and the dummy bit line 19 is not likely to occur.
Various other modifications will be apparent to and can be readily made by
those skilled in the art without departing from the scope and spirit of
this invention. Accordingly, it is not intended that the scope of the
claims appended hereto be limited to the description as set forth herein,
but rather that the claims be broadly construed.
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